Articles, such as absorbent articles, are useful for absorbing many types of fluids, including fluids secreted or eliminated by the human body. Superabsorbent polymers (SAPs) are frequently used in absorbent articles to help improve the absorbent properties of such articles. SAPs are generally polymer based and are available in many forms, such as powders, granules, microparticles, films and fibers, for example. Upon contact with fluids, such SAPs swell by absorbing the fluids into their structures. In general, SAPs can quickly absorb fluids insulted into such articles, and can retain such fluids to prevent leakage and help provide a dry feel even after fluid insult.
There is a continuing effort to improve the performance of absorbent articles, especially at high levels of fluid saturation, to thereby reduce the occurrence of leakage and to improve fit and comfort. This is particularly significant when such articles are subjected to repeated fluid insults during use. This has become an increasing challenge as recent efforts in absorbent article design have generally focused on using higher concentrations of superabsorbent material and less fluff fibers to make the absorbent structures thinner and more flexible. However, notwithstanding the increase in total absorbent capacity obtained by increasing the concentration of superabsorbent material, such absorbent articles may still nevertheless leak during use. Such leakage may in part be the result of the absorbent core component of an article having an insufficient intake rate (i.e., the rate at which a fluid insult can be taken into and entrained within the absorbent core for subsequent absorption by the superabsorbent material) due to factors such as low permeability and lack of available void volume. Therefore, there is a desire for an absorbent article which contains high levels of superabsorbent materials and which can maintain a sufficient intake rate.
In addition, there is also a need for superabsorbent polymer materials that have increased permeability characteristics while retaining other characteristics such as adequate absorption and/or retention. Permeability is a measure of the effective connectedness of a porous structure, be it a mat of fiber or a slab of foam or, in this case, crosslinked polymers and may be specified in terms of the void fraction and extent of connectedness of the superabsorbent polymer material. Gel permeability is a property of the mass of particles as a whole and is related to particle size distribution, particle shape, and the connectedness of the open pores, shear modulus and surface modification of the swollen gel. In practical terms, the permeability of the superabsorbent polymer material is a measure of how rapidly liquid flows through the mass of swollen particles. Low permeability indicates that liquid cannot flow readily through the superabsorbent polymer material, which is generally referred to as gel blocking, and that any forced flow of liquid (such as a second application of urine during use of the absorbent article) must take an alternate path (e.g., leakage). Therefore, there is a need for an absorbent article which exhibits improved permeability characteristics.
One method to increase permeabilities in extremely thin diapers with low fiber content is to increase the amount of crosslinking of the superabsorbent material. However, the absorption and retention values of the superabsorbent polymer compositions may then be reduced to undesirably low levels when the crosslinking of the superabsorbent polymer is increased. Therefore, there is a further need to provide superabsorbent polymer compositions that exhibit improved application properties including a high absorption capacity to retain fluids under no load, high absorption capacities to retain fluid under pressure, and/or improved gel bed permeability.